This invention relates generally to counter-top ovens for domestic or commercial use, and in particular to an oven capable of carrying out all of the usual food baking, defrosting and other cooking functions of ovens of the conventional type and also adapted to effect low-temperature cooking of food at a much faster rate than standard ovens.
The term "counter-top oven," as used herein, is intended to apply to any oven which is sufficiently compact to be placed on top of a table or counter, but the term is not limited to ovens so placed, and the heat transfer principles involved are applicable to any food cooking oven regardless of its size.
A multi-function counter-top electrical oven such as the Proctor-Silex Meal Maker II manufactured by SCM Corporation of Baltimore, Md., is adapted to bake, broil and slow-cook food products and to carry out other food heating functions. Ovens of this type are provided with thermostatically-controlled resistance heater elements operating in conjunction with an adjustable timer to generate oven heat at a predetermined level for a preset time period appropriate to the nature of the intended cooking action, such as baking or broiling. The "Versatron Electronic Countertop Oven" marketed by GE is further adapted to top brown as well as to cook food, the necessary high temperatures for this function being produced by quartz tube heating elements.
In order to bring about a uniform distribution of heat and thereby avoid hot spots that would otherwise burn the food, the oven disclosed in U.S. Pat. No. 3,828,760 of Faber et al. is provided with a forced-air convection system in which the heated air is caused to assume a cyclonic turbulent pattern. This oven is intended for broiling, roasting and baking and may be used for defrosting as well.
The boiling point of water is 212.degree. F., whereas food typically undergoes cooking when its internal temperature is below this boiling point; that is, at about 140.degree. to 200.degree. F. Yet the common practice with conventional ovens is to establish an oven temperature that runs as high as 500.degree. F. Thus in the "Oven Guide" attached to the "Versatron" oven above-identified, the oven temperature for cooking frozen dinners is given as 425.degree. F., for baking biscuits as 450.degree. F., and for keeping food warm or for reheating pre-cooked food as 300.degree. F.
The reason why in a conventional oven, the interior temperature is so much above the temperature at which food cooks is that the rate of heat transfer between the hot oven atmosphere and the body of the food is high only when the temperature differential therebetween is great.
If, therefore, the food placed in the oven is initially at a temperature of 70.degree. F. and the oven temperature is held at 220.degree. F., then as the body of food becomes warmer and its surface temperature reaches, say, 150.degree. F., the rate of heat transfer thereafter becomes increasingly sluggish, so that it would literally take many hours to fully cook the food.
On the other hand, when the temperature differential is great, say, with an oven atmosphere at a temperature of 400.degree. F. and a food surface temperature initially at 70.degree. F., at no time in the course of cooking is the temperature differential small. The cooking rate is therefore quite fast even as the body temperature approaches the cooking temperature. However, with these conventional ovens in which a large heat differential exists, there is always the danger of overcooking or burning the food.
Because the outer layer of the food body is in direct heat transfer relation with the oven atmosphere whereas the intermediate layers and core are, by their very nature, thermally-insulated therefrom by the outer layer, in order to heat up and cook the intermediate layers and core, it is necessary to somewhat overcook or possibly burn the outer layer. Hence if the food is withdrawn from the oven when the outer layer appears to be adequately cooked, the food body may then be insufficiently cooked at the core and at the intermediate layers.
The deleterious effects of high-temperature cooking on food is well recognized, for such cooking is destructive of nutrients, vitamins and valuable soluble constituents, and tends to rob food of its taste and flavor. The preferable practice is, therefore, to cook food at a low temperature, this being traditionally accomplished with an oven temperature not much higher than the cooking temperature of the food.
With conventional low-temperature cooking, the nutrients and other valuable constituents are preserved and the food is much more flavorsome than when subjected to high temperature cooking. But with counter-top ovens of the type heretofore known, a heavy penalty is paid when operating the oven in the low-temperature mode, for the resultant small heat differential is such that it takes as much as 8 hours or longer to cook the food.
This explains why in the "Use and Care" manual published for the Proctor-Silex oven, low temperature cooking is referred to therein as "slow cooking" or as a "low temperature slow cook;" for prior to the present invention, low temperature cooking was invariably very slow. Hence while the advantages gained by low-temperature cooking are appreciated by most consumers, many of them simply cannot afford the time it takes and therefore use their counter-top ovens only for high-temperature fast cooking. This problem is especially bothersome when dealing with pre-cooked frozen meals.
Another drawback of conventional ovens, particularly those capable of low-temperature slow cooking, is that because these ovens are usually provided with uninsulated metal housings that are thermally conductive and discharge heat into the room, much of the energy required to carry out cooking is wasted during the prolonged cooking period. Also, an oven having a high housing temperature represents a hazard, for it can seriously burn an individual who touches the housing.